The present invention relates to a device for calibrating a parallel kinematic manipulator, and more specifically to a device suitable for calibrating a machine tool designed according to the hexapod principle.
Alternative machine tool kinematics are being increasingly used in designing machine tools, such as in machine tools designed according to the hexapod principle or as Stewart platforms. In such machine tools, or more generally in parallel kinematic manipulators, the relative motion between workpiece and tool is effected by the simultaneous motion of actuators normally moving in six translational and/or rotational axes. The respective actuators normally consist of variable-length telescopic arms.
In order to ensure the required positional accuracy, the geometry of the parallel kinematic manipulator being used must be known with extreme precision. In general, the actual geometry of such a manipulator is not identical to the designed ideal geometry due to manufacturing and assembly tolerances. These deviations in turn result in accuracy deviations when the machine is used in machining a workpiece. Therefore, such a parallel kinematic manipulator must be calibrated prior to use to determine its actual geometry. A correction can then be used when machining the workpiece to account for the deviation in manipulator geometry.
In particular, in the case of a typical design consisting of one base unit connected to a manipulator platform via a pluralityxe2x80x94for example, sixxe2x80x94parallel actuators, the exact coordinates of the joints between the actuators and the base unit/manipulator platform must be known. Only when these coordinates are known can the tool and thus the xe2x80x9ctool center point,xe2x80x9d hereinafter referred to as TCP, be definitively positioned on the manipulator platform with the required accuracy. The TCP is the point of the tool that interacts with the workpiece.
To calibrate such kinematic manipulators, a plurality of calibration algorithms have been proposed. In all these calibration methods, a certain number of TCP points are measured in the manipulator""s work space, from which the actual joint coordinates are determined. The joint coordinates are in turn taken into account by the controller in positioning the tool. Reference is made to the following publications, that describe examples of he calibration procedure.
i) Global Kinematic Calibration of a Stewart Platform, Oliviers M. P., Mayer J. R. R., Proceedings of the ASME Dynamic Systems and Control Division, DSC Vol. 57-1; 1995, pp. 129-136;
ii) Algorithms for Kinematic Calibration of Fully-Parallel Manipulators, Innocenti C., Computational Kinematics (Edts: Merlet, Ravani), Kluwer Academic Publishers 1995, pp. 241-250;
iii) A 3D Sensor for Parallel Robot Calibration. A Parameter Perturbation Analysis, Fried G. et al., Recent Advances in Robot Kinematics (Edts: Lenarcic, Parenti-Castelli), Kluwer Academic Publishers 1996, pp. 451-460;
iv) Volumetric Error Analysis of a Stewart Platform-based Machine Tool, Patel A., Ehmann K., Annals of the CIRP, Vol. 46/1/1997.
In these publications, calibration algorithms for parallel kinematic manipulators are presented and discussed. However, no practical calibration device for those calibration algorithms, offering sufficient accuracy, is provided by any of these publications.
Accordingly, there is a need for an improved device for calibrating parallel kinematic manipulators that obviates some of the drawbacks of currently known devices.
The present invention is directed to a device that allows a parallel kinematic manipulator to be calibrated with a high degree of accuracy, and that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. Other advantages of the invention will be realized and obtained by the apparatus and method particularly pointed out in the written description and claims hereof, as well as the appended drawings.
According to one embodiment, the invention is a calibration device for measuring n deviations and determining m unknown spatial coordinates of a parallel kinematic manipulator having a base unit and a manipulator platform movable with respect to the base unit, the base unit being connected to the manipulator platform via a plurality of joints and a plurality of variable-length actuators, and having a workholding fixture for a tool. The calibration device includes a test workpiece mountable in a defined spatial relation to the base unit, measuring mandrels numbering l=m/n, arranged on the test workpiece in a known spatial position and a known spatial orientation with respect to a reference spatial position and a reference spatial orientation, and a probing unit mountable on the workholding fixture of the manipulator platform. The calibration device also includes n probing subsystems, each of the n probing subsystems including a distance measuring device for relative measurements, and being oriented with respect to each other of the n probing subsystems such that the probing unit is adapted for measuring the known spatial position and known spatial orientation of the measuring mandrels on the test workpiece.
In another embodiment, the invention is a method of calibrating a parallel kinematic manipulator that includes selecting a test workpiece comprising a plurality of measuring mandrels arranged in a known spatial position and known spatial orientation, and a reference measuring mandrel defining a reference spatial position and orientation, selecting a probing unit having a plurality of probing subsystems for measuring relative distances, the probing unit being attached to a manipulator platform movable relative to a base unit of the kinematic manipulator, and positioning the test workpiece a defined spatial relation to the base unit. The method further includes determining measured spatial position and measured spatial orientation of the plurality of measuring mandrels and the reference measuring mandrel with the probing unit, computing calibration values for the kinematic manipulator from the known spatial position and orientation and from the measured spatial position and orientation of the plurality of measuring mandrels and the reference measuring mandrel.
The calibration device according to the present invention allows a parallel kinematic manipulator to be calibrated in a simple manner. The device according to the present invention can also be used in combination with a plurality of manipulator geometries, and is therefore extremely adaptable.
Different calibration algorithms from the one described in detail can also be used in conjunction with the device according to the present invention, since the device allows several translational and/or rotational deviations to be simultaneously measured in a single contact probing operation.
The device according to the present invention allows the actual TCPs to be determined or measured with a high degree of accuracy. Thus, a suitably designed parallel kinematic manipulator can also ultimately be used to position a machine tool.
The device according to the present invention has the further advantage that it is essentially made up of simple standard components, with the result that manufacturing costs are low. In particular, the test workpiece of the calibration device does not have to be designed with high geometric precision. It is only necessary that the exact geometry of the test workpiece be determined prior to use, for example, by measuring it with a coordinate measuring device.
Further advantages and details of the calibration device according to the present invention, and a suitable calibration algorithm for the device are provided in the description of an embodiment that follows, with reference to the attached drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.